A rock that just missed us

Gaspra's very irregular shape suggests the asteroid came from a larger body by what was nearly a catastrophic collision. The large concavity on the lower right limb is about 6 kilometers (3.7 miles) across. The prominent crater on the terminator (center left) is about 1.5 kilometers (1 mile) across. A striking feature of Gaspra's surface is the abundance of small craters. More than 600 craters 100-500 meters (330-1650 feet) in diameter are visible in this image.

NEAR / JPL / NASA

August 23, 2004On March 31, 2004, a small asteroid, designated 2004 FU162, skimmed past Earth at an altitude of about 4,000 miles (6,500 kilometers), or roughly one Earth radius. The close passage was announced by scientists on August 22, long after the object was discovered and tracked by the LINEAR asteroid search. Based in New Mexico, LINEAR observed the asteroid four times over a 44-minute span before the rock made its closest approach to Earth. Attempts to follow up these observations failed, however, because the asteroid moved into the daylight sky and was lost. A search for earlier observations proved fruitless.

"I was aware of the four observations immediately because the Minor Planet Center wanted my opinion on whether an impact was possible," says astronomer Steve Chesley of NASA's Jet Propulsion Laboratory, who calculated FU162's close approach. "I've been working on the object ever since with new techniques that I developed."

Chesley took the four observations and computed an orbit for the object — or actually two orbits, one for before the space rock's Earth encounter and one for after. Before flying past Earth, FU162 had a year-long orbit that straddled Earth's path about evenly. During the flyby, Earth's gravity bent the rock's trajectory. "It deflected it by about 20°," Chesley told Astronomy. "The asteroid's period dropped from about one year to about 9 months." Its new orbit lies closer to the Sun than the older one.

Only 15 to 30 feet (5 to 10 meters) in diameter, 2004 FU162 produced the closest miss of any asteroid known. If it had struck Earth, its size was small enough to make it disintegrate high in the atmosphere. Says Chesley, "It would have produced a nice fireworks show." He continues: "Objects this size hit Earth every several years."

The previous close-approach record holder was 2004 FH, which flew past Earth at a distance of 27,000 miles (43,000 km) on March 18, 2004. It had a diameter estimated at about 100 feet (30m). That object would have made a larger explosion, but it too would have broken up in the atmosphere if it had struck.